Optical device and movable reflector

ABSTRACT

[Problem] To reduce optical feedback to an optical path.  
     [Solution] Variable optical attenuator  100  includes optical path  26  having optical axis  16 , optical path  27  having optical axis  17 , and mirror  21  arranged to move in directions  32, 33  across bisector  18  of an angle between optical axes  16  and  17 . Mirror  21  has a surface including reflecting portion  21   a  for reflecting light from optical path  11  toward optical path  12 . Reflecting portion  21   a  has an edge  21   b  including a linear portion placed on a plane substantially perpendicular to bisector  18 . The linear portion is inclined relative to the normal to a plane including optical axes  16, 17.

TECHNICAL FIELD

The present invention relates to an optical device for regulating thepower of light propagating from a first optical path to a second opticalpath, and a movable mirror device used in the optical device.

BACKGROUND ART

Optical communications are often performed using optical devices, e.g.,variable optical attenuators or optical switches, for adjusting thepower of optical signals propagating in optical waveguides. In anexample of such optical devices, a mirror is placed in an optical pathbetween two optical waveguides, and the mirror is moved to vary thequantity of light reflected by the mirror, thereby adjusting the powerof the light fed from one of the optical waveguides into the otheroptical waveguide (see Non-patent Document 1).

[Non-patent Document 1] “Micro-Opto-Mechanical 2×2 Switch for SingleMode Fibers based on Plasma-Etched Silicon Mirror and ElectrostaticActuation” (preceding 11th IEEE Workshop on Micro-Electro-MechanicalSystem, 1998, pp233-237).

DISCLOSURE OF THE INVENTION

[Problem to be Solved by the Invention]

FIG. 10 is a schematic plan view showing an example of a variableoptical attenuator using a mirror. The variable optical attenuator 50has a Planar Lightwave Circuit (PLC) 10, a mirror 20, and a mirrordriver device 30. Optical waveguides 11 and 12 in the PLC 10 have theirrespective ends placed in mirror symmetry with respect to a plane 13.These ends have their respective end faces 11 a and 12 a aligned on asingle plane. The mirror 20 has a reflecting surface 20 a parallel tothese end faces 11 a and 12 a. The mirror driver device 30 is able tomove the mirror 20 along directions indicated by arrows 32 and 33. Whenlight from the optical waveguide 11 is incident onto the reflectingsurface 20 a, it is reflected toward the optical waveguide 12. Thiscauses the light to propagate from the optical waveguide 11 to theoptical waveguide 12. On the other hand, when the light from the opticalwaveguide 11 is not incident onto the reflecting surface 20 a, the lightdoes not enter the optical waveguide 12.

As shown in FIG. 10, the reflecting surface 20 a has an edge 20 b thatmoves across the plane 13 with movement of the mirror 20. At the edge 20b incident light is scattered into various directions by virtue ofdiffraction. For this reason, part of the light from the opticalwaveguide 11 is fed back to the optical waveguide 11 to propagate againin the optical waveguide 11. This light is optical feedback to theoptical waveguide 11. This optical feedback can distort the waveform ofthe signal light propagating in the optical waveguide 11 and cause acommunication error.

It is, therefore, an object of the present invention to reduce theoptical feedback to a first optical path in an optical device forregulating the power of light propagating from the first optical path toa second optical path.

[Means for Solving the Problem]

FIG. 11 shows the relationship between the position of the mirror edge20 b and coupling efficiency in the variable optical attenuator 50 shownin FIG. 10. At the mirror edge position of 0 μm, the edge 20 b islocated on the plane 13 between the optical waveguides 11 and 12. InFIG. 11 a solid line represents the coupling efficiency of lighttravelling from the optical waveguide 11 to the optical waveguide 12, achain line the coupling efficiency of light fed from the opticalwaveguide 11 back to the optical waveguide 11, and a chain double-dashedline the coupling efficiency of light fed from the optical waveguide 12back to the optical waveguide 12. It is noted that in FIG. 11 the chainline and the chain double-dashed line overlap each other. As shown inFIG. 11, the variable optical attenuator 50 demonstrates the largecoupling efficiency of optical feedback to the optical waveguides 11 and12. Therefore, the waveform of signal light in the optical waveguides isrelatively likely to be distorted.

A conceivable method of preventing the distortion of the waveform ofsignal light is to connect isolators 51 and 52 to the respective opticalwaveguides 11 and 12, as shown in FIG. 12. Signal light 55 propagatingin the optical waveguide 11 is reflected by the mirror 20 and is thenincident into the optical waveguide 12 to propagate in the opticalwaveguide 12. Optical feedback 56 to the optical waveguide 11, caused byscattering at the edge part 20 b of the mirror 20, is cut off by theisolator 51 connected to the optical waveguide 11. The isolator 52connected to the optical waveguide 12 cuts off optical feedback 57 froman external device connected to the variable optical attenuator 50 toprevent incidence thereof into the variable optical attenuator 50.Therefore, it is also feasible to prevent occurrence of optical feedbackfrom the optical waveguide 12 to the optical waveguide 12. A typicaltolerance of coupling efficiency of optical feedback is −45 dB, andtolerances differ according to systems using the variable opticalattenuator.

The use of isolators as described above makes it feasible to prevent theinfluence of optical feedback on the signal light in the opticalcommunication system using the variable optical attenuator. However, theneed for connection of the isolators to the optical waveguides leads tomaking construction of the system more complex and increasing theproduction cost of the system. Hence, the Inventors invented anotheroptical device and mirror capable of reducing the optical feedback.

In one aspect, the present invention relates to an optical devicecomprising a first optical path having a first optical axis; a secondoptical path having a second optical axis not parallel to the firstoptical axis; and a mirror adapted to move across a bisector of an anglebetween the first optical axis and the second optical axis. The mirrorhas a surface including a reflecting portion for, when receiving lightfrom the first optical path, reflecting the light toward the secondoptical path.

The reflecting portion may have an edge including a linear portionplaced on a plane substantially perpendicular to the bisector. Thelinear portion may be inclined relative to a normal to a plane includingthe first and second optical axes. In another configuration, thereflecting portion may have an edge including a curved portion placed ona plane substantially perpendicular to the bisector.

The linear portion and the curved portion are not parallel to the normalto the plane including the first and second optical axes. Thesenon-parallel portions on the edge of the reflecting portion reduces acomponent in the first axis direction of scattered light that occurswhen a light from the first optical path enters these portions.

To further reduce the optical feedback to the first optical path, theabove acute angle φ is preferably not less that 5°.

In still another configuration, the reflecting portion may have an edgeincluding a portion placed on a plane substantially perpendicular to thebisector, and in this portion, the value of function Rav(X) defined bythe following equation varies at least from 10% to 90% between twodifferent X coordinates:Rav(X)=∫R(X,Y)·φ(Y)dY/∫φ(Y)dY,where X represents a coordinate in an X-axis direction extending alongan intersecting line between the plane including the first and secondoptical axes, and the reflecting portion, Y a coordinate in a Y-axisdirection extending perpendicularly to the X-axis on the reflectingportion, R(X,Y) a reflectance distribution on the XY plane, and φ(Y) aY-directional intensity distribution of light incident from the firstoptical path to the reflecting portion.

A portion on the edge of the reflecting portion of the mirror, whichportion provides function Rav (X, Y) having such a distribution, reducesa component in the first axis direction of scattered light that occurswhen a light from the first optical path enters the portion. Thisdecreases the optical feedback to the first optical path.

To further reduce the optical feedback to the first optical path, thedistance between the two X coordinates where the value of functionRav(X) varies from 10% to 90% is preferably not less than 3% of a modefield diameter in the X-direction of the light incident from the firstoptical path to the reflecting portion.

The optical device in accordance with the present invention may furthercomprise at least either an optical waveguide optically coupled to thefirst optical path or an optical waveguide optically coupled to thesecond optical path. The optical waveguides may be planar waveguides oroptical fibers.

In another aspect, the present invention relates to a movable mirrordevice comprising a reflecting surface, and a driver device capable ofmoving the reflecting surface along a predetermined moving path. Themoving path extends in parallel with a plane substantiallyperpendicularly traversing the reflecting surface. The reflectingsurface has an edge adapted to move while intersecting the plane as thereflecting surface moves along the moving path. The edge may include alinear portion inclined relative to a normal to the plane. An acuteangle between the linear portion and the normal is preferably not lessthan 5°. In another configuration, the edge may include a curvedportion.

The linear portion and the curved portion are not parallel to the normalto the plane parallel to the moving path of the reflecting portion.These non-parallel portions on the edge of the reflecting portionreduces a component of scattered light in a direction of an optical axisplaced on the above plane, which scattered light occurs when a light onan optical path having the optical axis enters the non-parallelportions.

[Advantageous Effect of the Invention]

According to the invention, it is possible to reduce optical feedback toa first optical path in an optical device for adjusting the power oflight propagating from the first optical path to a second optical path.

[Best Mode for Carrying out the Invention]

The preferred embodiments of the present invention will be describedbelow in greater detail with reference to the accompanying drawings. Tofacilitate understanding, identical reference numerals are used, wherepossible, to designate identical or equivalent elements that are commonto the embodiments, and, in subsequent embodiments, these elements willnot be further explained.

(First Embodiment) FIG. 1 is a schematic plan view showing an opticaldevice of the first embodiment. This optical device is a variableoptical attenuator 100. The variable optical attenuator 100 has a PlanarLightwave Circuit (PLC) 10, a mirror 21, and a mirror driver device 30.The mirror 21 and the mirror driver device 30 constitute a movablemirror device 40.

The PLC 10 has two optical waveguides 11 and 12. The optical waveguides11 and 12 are planar waveguides extending in parallel to the plane ofFIG. 1. The optical waveguides 11 and 12 are made, for example, ofsilica glass. The ends of the optical waveguides 11 and 12 on the sidenear the mirror 21 may intersect and overlap with each other, or may beseparated from each other.

The mirror 21 is an optical reflector having a reflecting surface 21 a.The reflecting surface 21 a is substantially flat and has an extremelyhigh reflectance (e.g., 90% or more) for light of a predeterminedwavelength propagating in the optical waveguides 11 and 12. Thereflecting surface 21 a has a substantially uniform reflectance. Thereflecting surface 21 a is provided on a surface of the mirror 21 andextends in the direction perpendicular to the plane of FIG. 1. Themirror 21 moves so that the reflecting surface 21 a faces the end facesof the optical waveguides 11 and 12. The clearance between thereflecting surface 21 a and these end faces may be filled with arefractive-index matching material. The details of the mirror 21 will bedescribed later.

The mirror driver device 30 moves the mirror 21 substantially inparallel with the ZX plane, as indicated by arrows 32 and 33. Inresponse thereto, the reflecting surface 21 a of the mirror 21 movesalong a moving path 46. The movement of the mirror 21 is reversible. Themoving path 46 extends in parallel with a plane (e.g., the plane ofFIG. 1) substantially perpendicularly traversing the reflecting surface21 a. In the present embodiment the moving path 46 is of a linear shapeextending substantially in the X-direction. In the region near the endfaces of the optical waveguides 11 and 12, therefore, the reflectingsurface 21 a moves substantially in parallel with the end faces of theoptical waveguides 11 and 12, as indicated by arrows 32 and 33. Anexample of the mirror driver device 30 is an electrostatic actuator asdescribed in non-patent document 1 mentioned above.

The moving path 46 may also be of a curved shape. If the radius ofcurvature of the moving path 46 is sufficiently large, the reflectingsurface 21 a can be moved substantially in the X-direction in the regionnear the end faces of the optical waveguides 11 and 12.

An XYZ orthogonal coordinate system is depicted in FIG. 1 forconvenience' sake of description. The X-axis extends along anintersecting line between the reflecting surface 21 a and the planeincluding both the optical axes of the waveguides 11 and 12. The Y-axisextends perpendicularly to the X-axis in the plane perpendicular to abisector of an angle between the two optical axes of the waveguides 11and 12. The Z-axis extends in parallel with the bisector.

The mirror 21 will be described below in further detail with referenceto FIGS. 2 and 3. FIG. 2 is a schematic perspective view showing themirror 21. FIG. 3 is a view showing the reflecting surface 21 a of themirror 21 from an angle different from that in FIG. 2.

As shown in FIG. 2, the variable optical attenuator 100 has opticalpaths 26 and 27 for feeding light from the optical waveguide 11 via themirror 21 to the optical waveguide 12. The optical paths 26 and 27 areoptically coupled to the optical waveguides 11 and 12, respectively. Thereflecting surface 21 a of the mirror 21 moves so as to intersect theoptical path 26. The optical paths 26 and 27 extend between thereflecting surface 21 a and the end faces of the optical waveguides 11and 12. Light 41 emerging from the optical waveguide 11 travels on theoptical path 26 toward the mirror 21, and is reflected on the reflectingsurface 21 a to travel on the optical path 27 toward the opticalwaveguide 12. The optical paths 26 and 27 have their respective opticalaxes 16 and 17. The optical axes 16 and 17 are placed on a planeparallel to the plane of FIG. 1. The optical axes 16 and 17 are notparallel to each other, and intersect at an angle θ. A bisector 18 ofthe angle θ extends on the plane including the optical axes 16 and 17.Numeral 14 in FIG. 2 represents the plane including the optical axes 16and 17. The plane 14 will be referred to hereinafter as a referenceplane. The reference plane 14 is substantially parallel to the movingpath 46 of the reflecting surface 21 a and substantially perpendicularlytraverses the reflecting surface 21 a.

As shown in FIG. 2, the reflecting surface 21 a of the mirror 21 istrapezoidal. The reflecting surface 21 a has a linear edge 21 b arrangedto move so as to pass across the bisector 18 with movement of the mirror21. The edge 21 b moves while intersecting the reference plane 14 as thereflecting surface 21 a moves along the moving path 46. The reflectingsurface 21 a and the edge 21 b are located substantially on the XYplane. The bisector 18 is parallel to the Z-axis, as described above.Therefore, the edge 21 b is located on a plane substantiallyperpendicular to the bisector 18. The edge 21 b is inclined relative toa normal 15 to the reference plane 14 and makes an acute angle φ withthe normal 15.

In order to efficiently couple light between the optical waveguides 11and 12 via the reflecting surface 21 a, the reflecting surface 21 a andthe edge 21 b are preferably perfectly perpendicular to the bisector 18.In practice, however, a satisfactorily high coupling efficiency can beachieved if an angle between a projected line of the bisector 18 ontothe reflecting surface 21 a and the bisector 18 is within the range of85° to 90°, and more preferably within the range of 89° to 90°.

When receiving from the optical waveguide 11 the light 41 travellingalong the optical axis 16 of the optical path 26, the reflecting surface21 a reflects the light 41 along the optical axis 17 of the optical path27. In consequence, the light 41 from the optical waveguide 11 isincident along the optical axis 17 into the optical waveguide 12 andthen propagates in the optical waveguide 12. On the other hand, if thelight from the optical waveguide 11 does not impinge on the reflectingsurface 21 a, the light is not incident to the optical waveguide 12.

FIG. 3 shows an enlarged view of a cross section of beam 44 of incidentlight 41 When the light incident from the optical waveguide 11 to thereflecting surface 21 a is distributed on the edge 21 b, as shown inFIG. 6, the incident light is scattered at the edge 21 b by virtue ofdiffraction. Part of the scattered light is coupled to the opticalwaveguide 12 and propagates in the optical waveguide 12. After themirror 21 is moved in the direction of arrow 32 from the position shownin FIG. 6, the incident light comes to be reflected by a narrower regionon the reflecting surface 21 a, so as to decrease the couplingefficiency from the optical waveguide 11 to the optical waveguide 12.Conversely, after the mirror 21 is moved in the direction of arrow 33from the position shown in FIG. 3, the incident light comes to bereflected by a wider region on the reflecting surface 21 a, so as toincrease the coupling efficiency from the optical waveguide 11 to theoptical waveguide 12. Therefore, the power of light propagating from theoptical waveguide 11 to the optical waveguide 12 can be varied accordingto the movement of the mirror 21. This is the principle of the operationof the variable optical attenuator 100.

Part of the light scattered at the edge 21 b returns to the opticalwaveguide 11. This is optical feedback to the optical waveguide 11. Inthe present embodiment the optical feedback is reduced by theinclination of the edge 21 b relative to the normal 15 to the referenceplane 14. The reduction of optical feedback by the mirror 21 in thepresent embodiment will be described below in comparison with the mirror20 shown in FIG. 10, with reference to FIGS. 2 and 13.

FIG. 13 is a schematic perspective view showing the mirror 20 shown inFIG. 10. The reflecting surface 20 a of the mirror 20 has an edge 20 b.The edge 20 b is a linear portion parallel to the normal 15 to thereference plane 14. Namely, the edge 20 b is normal to the referenceplane 14. Theoretically, when light is incident to the edge 20 b alongthe optical axis 16 on the reference plane 14 substantiallyperpendicular to the edge 20 b, optical scattering occurs in the plane14 and the scattered light 43 travels along the plane 14. The reason whyno scattered light appears outside the plane 14 is that, in FIG. 13,light components reflected by the portion of the reflecting surface 20 alocated on the upper side of the reference plane 14 and light componentsreflected by the portion of the reflecting surface 20 a located on thelower side of the reference plane 14 cancel each other according to theHuygens' principle. When optical scattering occurs in the plane 14normal to the edge 20 b, part of scattered light 43 is relatively easilycoupled with the optical waveguide 11 having the optical axis 16 on thesame plane 14. The optical feedback to the optical waveguide 11 isinduced in this manner.

On the other hand, the mirror 21 of the present embodiment has the edge21 b inclined relative to the normal 15 to the reference plane 14, asshown in FIG. 5. Therefore, the reference plane 14 is not perpendicularto the edge 21 b. For this reason, even if light is incident to the edge21 b along the optical axis 16 on the reference plane 14, opticalscattering occurs within a plane not parallel to the reference plane 14.This results in decreasing the coupling efficiency of the scatteredlight to the optical waveguide 11 and thus decreasing the opticalfeedback to the optical waveguide 11.

The effect of the present embodiment will be confirmed below withreference to FIG. 4. FIG. 4 shows the relationship between theinclination angle φ of the edge 21 b and the coupling efficiency of theoptical feedback to the optical waveguide 11, for various values of theintersecting angle θ between the optical axes 16 and 17. It is assumedherein that the incident light 41 is a beam having the wavelength of1.55 μm in vacuum and having a Gaussian distribution with the center onthe optical axis 16. The lateral MFD (mode field diameter) of incidentlight 41 is 20 μm and the vertical MFD 10 μm. The lateral direction andthe vertical direction herein are defined as the major-axis directionand the minor-axis direction of elliptical section 44 of the incidentlight beam shown in FIG. 3, and these are equivalent to the X-directionand the Y-direction, respectively. It is also assumed that the clearancebetween the end faces of the optical waveguides 11 and 12 and thereflecting surface 21 a is filled with a refractive-index matchingmaterial having the refractive index of 1.45.

As shown in FIG. 4, the coupling efficiency of the optical feedbackdecreases as the inclination angle φ of the edge 21 b increase, underall the values of θ. The larger the angle θ, the greater the effect ofdecreasing the coupling efficiency. Particularly, when θ is 45° or more,the coupling efficiency of the optical feedback can be greatlysuppressed to −40 dB or less under the inclination angle φ of 5° ormore.

The angle between the end faces of the optical waveguides 11 and 12facing the mirror 21 is determined according to the angle θ between theoptical axes 16 and 17. In the case where planar waveguides are used asthe optical waveguides 11 and 12, as in the present embodiment, thecurvature of the optical waveguides 11 and 12 tends to be large if θ islarge. In this case, light can leak in the curved portions of theoptical waveguides 11 and 12, so as to raise the risk of occurrence ofloss. In the case where the leak light needs to be particularlysuppressed, an appropriate range of the angle θ is 30° or less, and theinclination angle φ of the edge 21 b is preferably 10° or more. In thepresent embodiment where the reflecting surface 21 a moves along themoving path 46 substantially parallel to the reference plane 14, theinclination angle φ is preferably 75° or less. Increase of the angle φwill also result in increase of the moving distance of the reflectingsurface 21 a necessary for changing the power of light propagating fromthe optical waveguide 11 to the optical waveguide 12 by a predeterminedamount. Therefore, a too large angle φ will make it difficult todownsize the variable optical attenuator 100.

In order to efficiently reflect the light 41 by the reflecting surface21 a, the length of the edge 21 b is preferably larger than the MFD ofthe incident light 41 in the direction along the edge 21 b.

(Second Embodiment) The second embodiment of the present invention willbe described below with reference to FIGS. 1 and 5. FIG. 5 is aschematic perspective view showing a mirror 22 used in the secondembodiment. The optical device of the present embodiment is a variableoptical attenuator obtained by replacing the mirror 21 in the variableoptical attenuator 100 shown in FIG. 1, with the mirror 22 shown in FIG.5. The variable optical attenuator of the present embodiment has thesame configuration as the variable optical attenuator 100 except themirror.

The mirror 22 is an optical reflector having a reflecting surface 22 a.The reflecting surface 22 a is substantially flat and has an extremelyhigh reflectance (e.g. 90% or more) for light of a predeterminedwavelength propagating in the optical waveguides 11 and 12. Thereflecting surface 22 a has a substantially uniform reflectance. Themirror 22 moves so as to face the end faces of the optical waveguides 11and 12. The clearance between the reflecting surface 22 a and these endfaces may be filled with a refractive-index matching material.

As shown in FIG. 5, the reflecting surface 22 a has an edge 22 b of acurved shape. The edge 22 b moves so as to pass across the bisector 18with movement of the mirror 22. The reflecting surface 22 a and the edge22 b are located substantially on the XY plane. The bisector 18 isparallel to the Z-axis, as described above. Therefore, the edge 22 b islocated on the plane substantially perpendicular to the bisector 18. Itis a matter of course that the edge 22 b is not parallel to the normal15 to the reference plane 14.

In order to efficiently couple the light between the optical waveguides11 and 12 via the reflecting surface 22 a, the reflecting surface 22 aand the edge 22 b are preferably perfectly perpendicular to the bisector18. In practice, however, a satisfactorily high coupling efficiency canbe achieved if an angle between a projected line of the bisector 18 ontothe reflecting surface 22 a and the bisector 18 is within the range of85° to 90°, and more preferably within the range of 89° to 90°.

When receiving from the optical waveguide 11 the light 41 travellingalong the optical axis 16 of the optical path 26, the reflecting surface22 a reflects the light 41 along the optical axis 17 of the optical path27. Ii consequence, the light 41 from the optical waveguide 11 isincident along the optical axis 17 into the optical waveguide 12 andthen propagates in the optical waveguide 12. On the other hand, wherethe light from the optical waveguide 11 does not impinge on thereflecting surface 22 a, the light is not incident to the opticalwaveguide 12.

When the light incident from the optical waveguide 11 to the reflectingsurface 22 a is distributed on the edge 22 b, the incident light isscattered at the edge 22 b by virtue of diffraction. Part of thescattered light is coupled to the optical waveguide 12 and propagates inthe optical waveguide 12. After the mirror 22 is moved in the directionof arrow 32 from the position shown in FIG. 5, the incident light comesto be reflected by a narrower region on the reflecting surface 22 a, soas to decrease the coupling efficiency from the optical waveguide 11 tothe optical waveguide 12. Conversely, after the mirror 22 is moved inthe direction of arrow 33 from the position shown in FIG. 3, theincident light comes to be reflected by a wider region on the reflectingsurface 22 a, so as to increase the coupling efficiency from the opticalwaveguide 11 to the optical waveguide 12. Therefore, the power of lightpropagating from the optical waveguide 11 to the optical waveguide 12can be varied according to the movement of the mirror 22. This is theprinciple of the operation of the variable optical attenuator of thepresent embodiment.

As shown in FIG. 5, the edge 22 b of curved shape is not perpendicularto the reference plane 14. For this reason, even if light is incident tothe edge 21 b along the optical axis 16 on the reference plane 14,optical scattering occurs within a plane not parallel to the referenceplane 14. This decreases the coupling efficiency of the scattered lightto the optical waveguide 11, so as to reduce the optical feedback to theoptical waveguide 11.

More generally, the edge of the mirror extending in curved shape withinthe plane normal to the bisector 18 always includes a portion extendingin a direction not perpendicular to the reference plane 14, regardlessof the specific shape of the edge. Therefore, at least part of thescattered light is generated within a plane not parallel to thereference plane 14. For this reason, the mirror having the edge ofcurved shape is more unlikely to couple the scattered light at the edgeto the optical waveguide 11 than the mirror 20 having the edge 20 bconfigured of only the straight line normal to the reference plane 14.Therefore, the use of the mirror having the edge of curved portionsuccessfully decreases the coupling efficiency of optical feedback.

(Third Embodiment) The third embodiment of the present invention will bedescribed below with reference to FIGS. 1 and 6. FIG. 6 is a schematicplan view showing a mirror 23 used in the third embodiment. The opticaldevice of the present embodiment is a variable optical attenuatorobtained by replacing the mirror 21 in the variable optical attenuator100 shown in FIG. 1, with the mirror 23 shown in FIG. 6. The variableoptical attenuator of the present embodiment has the same configurationas the variable optical attenuator 100 except the mirror.

The mirror 23 is an optical reflector having a reflecting surface 23 a.The reflecting surface 23 a is substantially flat and has an extremelyhigh reflectance (e.g. 90% or more) for light of a predeterminedwavelength propagating in the optical waveguides 11 and 12. Thereflecting surface 23 a has a substantially uniform reflectance. Themirror 23 moves so that the reflecting surface 23 a faces the end facesof the optical waveguides 11 and 12. The clearance between thereflecting surface 23 a and these end faces may be filled with arefractive-index matching material.

As shown in FIG. 6, the reflecting surface 23 a has an edge 23 b of asawtooth shape. The edge 23 b has a configuration in which linearportions 23 c and 23 d are alternately connected. In FIG. 6 the linearportions 23 c extend right downwardly and the linear portions 23 d rightupwardly. A bisector of an angle between two adjacent linear portions 23c and 23 d is parallel to the X-axis.

The edge 23 b moves so as to pass across the bisector 18 of the anglebetween the optical axes 16 and 17 with movement of the mirror 23. Thereflecting surface 23 a and the edge 23 b are located substantially onthe XY plane. The bisector 18 is parallel to the Z-axis. Therefore, theedge 23 b is located on the plane substantially perpendicular to thebisector 18. The normal 15 to the reference plane 14 is parallel to theY-axis. The linear portions 23 c and 23 d forming the edge 23 b all areinclined relative to the normal 15 to the reference plane 14. Theselinear portions 23 c and 23 d make an acute angle φ with the normal 15.

In order to efficiently couple light between the optical waveguides 11and 12 via the reflecting surface 23 a, the reflecting surface 23 a andthe edge 23 b are preferably perfectly perpendicular to the bisector 18.In practice, however, a satisfactorily high coupling efficiency can beachieved if an angle between a projected line of the bisector 18 on thereflecting surface 23 a and the bisector 18 is within the range of 85°to 90° and more preferably within the range of 89° to 90°.

In FIG. 6 each of sawteeth of the reflecting surface 23 a is representedby reference number 23 e. Each sawtooth 23 e has a height H along theX-direction. These sawteeth 23 e are arranged at intervals D along theY-direction.

When receiving from the optical waveguide 11 the light 41 travellingalong the optical axis 16 of the optical path 26, the reflecting surface23 a reflects the light 41 along the optical axis 17 of the optical path27. In consequence, the fight 41 from the optical waveguide 11 isincident along the optical axis 17 of the optical waveguide 12 into theoptical waveguide 12 and then propagates in the optical waveguide 12. Onthe other hand, where the light 41 from the optical waveguide 11 doesnot impinge on the reflecting surface 23 a, the light 41 is not incidentto the optical waveguide 12.

When the light incident from the optical waveguide 11 to the reflectingsurface 23 a is distributed on the edge 23 b, the incident light isscattered at the edge 23 b by virtue of diffraction. Part of thescattered light is coupled to the optical waveguide 12 and propagates inthe optical waveguide 12. It is assumed herein that the beam of incidentlight impinges in the region around the origin of the XYZ coordinatesystem shown in FIG. 6. After the mirror 23 is moved in the direction ofarrow 32 from the position shown in FIG. 6, the incident light comes tobe reflected by a narrower region on the reflecting surface 23 a, so asto decrease the coupling efficiency from the optical waveguide 11 to theoptical waveguide 12. Conversely, after the mirror 23 is moved in thedirection of arrow 33 from the position shown in FIG. 6, the incidentlight comes to be reflected by a wider region on the reflecting surface23 a, so as to increase the coupling efficiency from the opticalwaveguide 11 to the optical waveguide 12. Therefore, the power of lightpropagating from the optical waveguide 11 to the optical waveguide 12can be varied according to the movement of the mirror 23. This is theprinciple of the operation of the variable optical attenuator of thepresent embodiment.

Just as in the first embodiment, since the edge 23 b is configured ofthe linear portions 23 c and 23 d inclined relative to the normal 15 tothe ZX plane, i.e., reference plane 14, the optical feedback to theoptical waveguide 11 can be reduced. However, the scattered light beamsat the plurality of linear portions 23 c and 23 d interfere with eachother, and thus the condition for achieving the adequate reduction ofoptical feedback is different from that in the first embodiment.

In view of this, the reduction of optical feedback in the presentinvention will be described below from another aspect. The descriptionherein will be given using the function Rav(X) defined by the followingequation:Rav(X)=∫R(X,Y)·Φ(Y)dY/∫Φ(Y)dY  (1).As shown in FIG. 2, X represents a coordinate in the X-axis directionextending along the intersecting line between the reference plane 14 andthe reflecting surface of the mirror, and Y a coordinate in thedirection perpendicular to the X-axis in the plane parallel to thereflecting surface of the mirror. R(X,Y) is a reflectance distributionin the XY plane. It is assumed herein that the reflectance is 100% at Xand Y coordinates where the reflecting surface 23 a is present and thatthe reflectance is 0% at X and Y coordinates where the reflectingsurface 23 a is absent. Φ(Y) indicates a Y-directional intensitydistribution of the light incident from the optical waveguide 11 to thereflecting surface 23 a.

The function Rav(X) indicates a reflectance distribution in thereflecting surface averaged by the Y-directional distribution of theincident light beam. In this interpretation of the suppression ofoptical feedback using Rav(X), the optical feedback is determinedaccording to the distribution of Rav(X), regardless of the shape of theedge of the mirror. Therefore, the description will be first given usingthe mirror 21 of FIG. 2. FIG. 7 shows Rav(X) for the mirror 21 (see FIG.2) on the assumption that Φ(Y) is a Gaussian distribution with the MFD(mode field diameter) of 10 μm and the inclination angle φ of the linearportion 21 b is 0°, 20° or 45°. In FIG. 7 the origin of X-coordinates istaken at the position where Rav(X) is 50%.

As shown in FIG. 7, in the case of φ=0°, Rav(X)=0% in the region of X>0,and Rav(X)=100% in the region of X<0. Namely, Rav(X) is discontinuous atthe origin of X-coordinates. With φ=0°, the edge 21 b is no longerinclined relative to the normal 15 to the reference plane 14, and is astraight line parallel to the normal 15, as in the case of the edge 20 bshown in FIG. 13 On the other hand, in the cases of φ=20° and 45°,Rav(X) is continuous at the origin of X-coordinates and smoothly variesnear the origin.

As shown in FIG. 7, the variation of Rav(X) near the origin ofX-coordinates becomes slower with increase of the inclination angle φ.Therefore, considering that the optical feedback is less with the edge21 b which corresponds to φ>0° than with the edge 20 b which correspondsto φ=0°, a condition necessary for reducing the optical feedback isconsidered to be that the function Rav(X) slowly varies. In the case ofφ=0°, Rav varies from 0% to 100% at the single X-coordinate of X=0.According to this fact, the Inventors considers that the opticalfeedback to the optical waveguide 11 could be adequately reduced incomparison with the mirror 20 having the edge 20 b if Rav(X) varies atleast from 10% to 90%, more preferably from 0% to 100%, between twodifferent X-coordinates.

Since it is sufficient that the variation of Rav(X) is slower than inthe case of φ=0°, Rav(X) does not always have to continuously vary. Forexample, even in a case where Rav(X) varies stepwise from 10% to 90%between two X-coordinates, the reducing effect of optical feedback canbe adequately achieved. However, the steps are more preferably as smallas possible because the reducing effect of optical feedback becomesgreater.

An X-directional width where Rav(X) varies from 10% to 90% will bereferred to hereinafter as an edge width. By replacing theabove-described horizontal axis in FIG. 4 with a ratio of edgewidth/X-directional MFD of incident light beam, FIG. 4 can be redrawninto FIG. 8. It is assumed herein that the incident light beam has aGaussian distribution and its X-directional MFD is 20 μm andY-directional MFD 10 μm.

According to the interpretation of reduction of optical feedback usingRav(X), the optical feedback is determined according to the distributionof Rav(X), regardless of the shape of the edge of the mirror. Therefore,while FIG. 4 is acquired about the edge 21 b of the shape shown in FIG.2, FIG. 8 redrawn from it is also applicable to edges of other shapes.

As shown in FIG. 8, in the case where the angle θ between the opticalaxes 16 and 17 is 45° or more, the coupling efficiency of opticalfeedback can be greatly reduced to −40 dB or less in the range when thevalue on the horizontal axis in FIG. 8, i.e., (edge width/X-directionalMFD) is 0.03 or more. In the edge 23 b of sawtooth shape shown in FIG.6, the edge width is approximately equal to the height H of the sawteeth23 e. Therefore, the optical feedback can be largely reduced if theheight H of sawteeth 23 e is not less that 3% of the X directional MFD.

In the case where the optical waveguides 11 and 12 are planarwaveguides, as in the present embodiment, the curvature of the opticalwaveguides 11 and 12 tends to be large if θ is large. In the case wherethe leak light from the optical waveguides needs to be particularlyreduced, an appropriate range of the angle θ is 30° or less and theinclination angle φ of the edge 21 b is preferably 10° or more. In thiscase, the edge width is preferably 6% or more of the X-directional MFD.

A method of producing the mirror of the above embodiment will bedescribed below with reference to FIGS. 9 (a) and (b). FIGS. 9 (a) and(b) is schematic perspective views showing examples of the mirror 23.

As shown in FIG. 9 (a), the mirror 23 can be produced by processing oneend portion of a substrate 23 f into sawtooth shape and thereaftercoating the upper surface of the substrate 23 f with a material 23 ghaving a high reflectance. The other mirrors 21 and 22 can also beproduced in similar fashion by processing an edge portion of a substrateinto a desired shape and thereafter coating the upper surface of thesubstrate with a high-reflectance material.

The mirror 23 can also be produced by coating an upper surface of asubstrate 23 h with a high-reflectance material 23 i so that it has anedge portion of sawtooth shape, as shown in FIG. 9 (b). The othermirrors 21 and 22 can also be produced in similar fashion by coating anupper surface of a substrate with a high-reflectance material so that ithas a desired shape. In this process, a portion without the coating ofthe high-reflectance material exists on the upper surface of thesubstrate. In order to reduce reflection in this portion, it ispreferable to coat the upper surface of the substrate with thehigh-reflectance material after coating it with an antireflectioncoating, or to move the mirror in a refractive-index matching materialhaving the refractive index approximately equal to that of thesubstrate.

The present invention was described above in detail on the basis of theembodiments thereof. It is, however, noted that the present invention isnot limited to the above embodiments. The present invention can bemodified in various ways without departing from the spirit and scope ofthe invention.

The above embodiments show the variable optical attenuators as examplesof the optical device in accordance with the present invention. However,the present invention may be applied to any other optical device forchanging the power of light propagating from one optical path to anotheroptical path. For example, the variable optical attenuators of the aboveembodiments can reduce the power of the light propagating from theoptical waveguide 11 to the optical waveguide 12 to almost zero bymoving the mirror. Therefore, these variable optical attenuators can beused as 1×1 optical switches for switching on and off the lightpropagating from the optical waveguide 11 to the optical waveguide 12.

The optical devices of the above embodiments have the optical waveguidesas optical paths. However, the optical devices of the present inventionmay also comprise optical paths formed in media (e.g., air) by arbitraryoptics such as lenses, instead of the optical waveguides. The opticalwaveguides used as the optical paths are not limited to the planarwaveguides in the above embodiments, but may be any other opticalwaveguides, e.g., optical fibers.

In the above embodiments, the reflecting surface of the mirror is flat.However, the present invention may also adopt the reflecting surfaceincluding a curved portion.

In the above embodiments, the mirror linearly moves in the directionsperpendicular to the bisector 18. However, the movement of the mirrordoes not have to be linear. For example, it is also possible to adopt aconfiguration in which a mirror is fixed to one end of a straightrod-like arm and the mirror is moved by rotating the arm around theother end of the arm. In this case, the moving path of the mirror is acurve of approximately arcuate shape. If the radius of curvature of themoving path is sufficiently large, the moving path becomes approximatelylinear.

In the optical devices of the present invention, the thickness in thedirection perpendicular to the reflecting surface of the mirror isarbitrary. For example, the mirror may have a uniform thickness in thedirection perpendicular to the reflecting surface.

In the optical devices of the present invention, the mirror or themirror driver device may be produced by use of theMicro-Electro-Mechanical System (MEMS) technology. Examples of themirror driver device include an electrostatic actuator, anelectromagnetic actuator using the electromagnetic force, and anactuator using thermal deformation. For example, an electrostaticactuator has a movable electrode part and a stationary electrode partand a mirror is set on the movable electrode part. The movable electrodepart is moved by an electrostatic force generated between the electrodesto move the mirror correspondingly.

In the third embodiment the plurality of sawteeth have the same heightand width. However, the reflecting surface of the mirror may have aplurality of sawteeth different in height or width, or in both.

In the above embodiments, desired Rav(X) is obtained by the appropriateshape of the edge of the reflecting surface with uniform reflectance.However, instead thereof, desired Rav(X) may also be achieved by thedistribution of the reflectance of the reflecting surface. For example,the distribution of the reflectance may be implemented by changing thethicknesses of the high-reflectance material, with which the reflectingsurface is coated, depending on positions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing a variable optical attenuator ofa first embodiment.

FIG. 2 is a schematic perspective view showing a mirror in the firstembodiment.

FIG. 3 is a view showing a reflecting surface of the mirror from anangle different from that in FIG. 5.

FIG. 4 is a graph showing coupling efficiencies of optical feedbackaccording to edge angles.

FIG. 5 is a schematic perspective view showing a mirror in a secondembodiment.

FIG. 6 is a schematic plan view showing a mirror in a third embodiment.

FIG. 7 is a chart showing function Rav(X).

FIG. 8 is a graph showing coupling efficiencies of optical feedbackaccording to (edge width /MFD in X-directional).

FIG. 9 is schematic perspective views showing examples of the mirror inthe third embodiment.

FIG. 10 is a schematic plan view showing an example of a variableoptical attenuator.

FIG. 11 is a graph showing the relationship between position of an edgeof a mirror shown in FIG. 1, and coupling efficiency.

FIG. 12 is a schematic diagram showing one method of reducing opticalfeedback.

FIG. 13 is a schematic perspective view showing a mirror.

DESCRIPTION OF THE SYMBOLS

10: planar lightwave circuit (PLC), 11 and 12: optical waveguide, 14:plane including two optical axes, 15: normal to plane including twooptical axes, 16 and 17: optical axis, 18: bisector of angle between twooptical axes, 21-23: mirror, 21 a-23 a: reflecting surface, 21 b-23 b:edge, 23 c and 23 d: straight portion, 23 e: 26 and 27: optical path,30: mirror driver device, 32 and 33: moving direction of mirror, 40:movable mirror device, 46: moving path of mirror, 100: variable opticalattenuator.

1. an optical device comprising: a first optical path having a firstoptical axis; a second optical path having a second optical axis notparallel to the first optical axis; and a mirror adapted to move acrossa bisector of an angle between the first optical axis and the secondoptical axis, the mirror having a surface including a reflecting portionfor, when receiving light from the first optical path, reflecting thelight toward the second optical path, the reflecting portion having anedge including a linear portion placed on a plane substantiallyperpendicular to the bisector, and the linear portion being inclinedrelative to a normal to a plane including the first and second opticalaxes.
 2. The optical device according to claim 1, wherein an acute anglebetween the linear portion and the normal is not less than 5°.
 3. Anoptical device comprising: a first optical path having a first opticalaxis; a second optical path having a second optical axis not parallel tothe first optical axis; and a mirror adapted to move across a bisectorof an angle between the first optical axis and the second optical axis,the mirror having a surface including a substantially flat reflectingportion for, when receiving light from the first optical path,reflecting the light toward the second optical path, and the reflectingportion having an edge including a curved portion placed on a planesubstantially perpendicular to the bisector.
 4. An optical devicecomprising: a first optical path having a first optical axis; a secondoptical path having a second optical axis not parallel to the firstoptical axis; and a mirror adapted to move across a bisector of an anglebetween the first optical axis and the second optical axis, the mirrorhaving a surface including a substantially flat reflecting portion for,when receiving light from the first optical path, reflecting the lighttoward the second optical path, the reflecting portion having an edgeincluding a portion placed on a plane substantially perpendicular to thebisector, and in the portion placed on the plane substantiallyperpendicular to the bisector, the value of function Rav(X) defined bythe following equation varies at least from 10% to 90% between twodifferent X-coordinates:Rav(X)=∫R(X,Y)·Φ(Y)dY/∫(Y)dY, where X represents a coordinate in anX-axis direction extending along an intersecting line between the planeincluding the first and second optical axes and the reflecting portion,Y a coordinate in a Y-axis direction extending perpendicularly to theX-axis on the reflecting portion, R(X,Y) a reflectance distribution onthe XY plane, and Φ)(Y) a Y-directional intensity distribution of lightincident from the first optical path to the reflecting portion.
 5. Theoptical device according to claim 6, wherein the distance between twoX-coordinates where the value of function Rav(X) varies from 10% to 90%is not less than 3% of an X-directional mode field diameter of the lightincident from the first optical path to the reflecting portion.
 6. Theoptical device according to any of claims 1 to 5, further comprising atleast either an optical waveguide optically coupled to the first opticalpath or an optical waveguide optically coupled to the second opticalpath.
 7. A movable mirror device comprising: a reflecting surface; and adriver device capable of moving the reflecting surface along apredetermined moving path, the moving path extending in parallel with aplane substantially perpendicularly traversing the reflecting surface,the reflecting surface having an edge adapted to move while intersectingthe plane as the reflecting surface moves along the moving path, and theedge including a linear portion inclined relative to a normal to theplane.
 8. The movable mirror device according to claim 7, wherein anacute angle between the linear portion and the normal is not less than5°.
 9. A movable mirror device comprising: a reflecting surface; and adriver device capable of moving the reflecting surface along apredetermined moving path, the moving path extending in parallel with aplane substantially perpendicularly traversing the reflecting surface,the reflecting surface having an edge adapted to move while intersectingthe plane as the reflecting surface moves along the moving path, and theedge including a curved portion.